Generally, the following three types are considered as a configuration of a transmitter used in a wireless communication terminal apparatus. (1) An architecture for mixing a base band signal with a local signal having the same frequency as a transmission frequency in a modulator. (2) An architecture for, after temporarily upconverting a base band signal to an intermediate frequency in a modulator, upconverting it to a transmission frequency by using a mixer. (3) An architecture for, after temporarily upconverting a base band signal to an intermediate frequency in a modulator, converting it to a transmission frequency in a PLL frequency conversion circuit.
In the architecture (3), since only a constant envelope modulation can be handled as a modulation architecture, architectures (1) and (2) have been mainstream as the architecture of the transmitter. However, since the GSM and the DCS1800 system rapidly widespread in recent years employ the constant envelope modulation as the modulation architecture, the architecture (3) having various advantages has been started to widely use. The advantages of the architecture (3) include: (1) that an expensive filter having a high Q value is not required in the transmitter according to filter characteristics which the PLL has, (2) that, since a VCO output signal is a constant envelope signal, a bias design of a power amplifier at the next stage of the PLL frequency conversion circuit becomes easy, and the like.
Here, the present inventors have investigated the transmitter according to the aforementioned architecture (3). The following is not a well-known technique, but a technique investigated by the present inventors, an outline thereof will be described with reference to FIGS. 6 to 9. FIG. 6 shows a transmitter of the architecture (3) according to a comparative example which is an assumption of the present invention. This transmitter is configured with a first frequency synthesizer 38, a second frequency synthesizer 39, a crystal oscillator 40 for giving reference signals to the first and second frequency synthesizers, a PLL frequency conversion circuit 41, a divider 47, a modulator 54, and a base band circuit 42.
The first frequency synthesizer 38 is configured with a first counter 42, a second counter 43, a phase comparator 44, a low pass filter 45, and a VCO 46, where an output signal of the VCO 46 is input into the divider 47.
The second frequency synthesizer 39 is configured with a third counter 48, a fourth counter 49, a phase comparator 50, a low pass filter 51, and a VCO 52, where an output signal of the VCO 52 which is a frequency fRF is input into a mixer 53.
On the basis of information signals such as a sound, various data, and the like, the base band circuit 42 is a circuit for generating a waveform of a base band signal or generating various data for controlling this transmitter.
Assuming a local signal output from the first frequency synthesizer 38 to be an input signal, the divider 47 divides this local signal into a frequency fIF, and inputs it into the modulator 54.
The modulator 54 mixes the signal of the frequency fIF supplied from the divider 47 into a base band signal from the base band circuit 42, and upconverts it to an intermediate frequency (for example, 270 MHz).
The PLL frequency conversion circuit 41 is configured with a phase comparator 55, a low pass filter 56, a VCO 57, and the mixer 53. Two signals are input into the phase comparator 55. A first input signal is an output signal of the modulator 54, and a second input signal is an output signal of the mixer 53. In the phase comparator 55, the first and second input signals are phase-compared with each other, and a signal proportional to a phase difference is output. The output signal of the phase comparator 55 is output to the low pass filter 56, where undesired noises are eliminated, and is then input into the VCO 57. The output frequency of the VCO 57 is an fVCO, which is used as an output signal of this transmitter and is input into the mixer 53. Two signals are input into the mixer 53. A first input signal is an output signal of the VCO 57, and a second input signal is a local signal of the frequency fRF supplied from the second frequency synthesizer 39. The output frequency of the mixer 53 is an absolute value of a difference between the two input frequencies, which becomes |fRF−fVCO|. The output signal of the mixer 53 becomes the second input signal of the phase comparator 55. Since, when the PLL frequency conversion circuit 41 is in a locked state, the two input frequencies of the phase comparator 55 are equal to each other, fIF=|fRF−fVCO| is obtained. Therefore, the output frequency fVCO of the VCO 57 is given as |fRF−fIF|. In other words, the output frequency fIF of the modulator 54 is frequency converted to fVCO=|fRF−fIF| in the output of the transmitter. In order to change the output frequency of the transmitter, the output frequency fRF of the second frequency synthesizer 39 is changed while the output frequency of the first frequency synthesizer 38 remains fixed.
Next, an example of closed-loop characteristics of the PLL frequency conversion circuit 41 is shown in FIG. 7. A flat portion of 0 dB is a bandpass. Since the frequency of the horizontal axis denotes a offset frequency from the output frequency fVCO, it is found that the PLL frequency conversion circuit 41 has bandpass filter characteristics around the output frequency. In other words, when the bandpass width is set to be wider than the bandwidth in the modulation architecture prescribed in the system such as the GSM or the like, the PLL frequency conversion circuit 41 can hold an output spectrum of the modulator 54, and convert the center frequency. Actually, in view of the phase difference and the noise level at the output of the PLL frequency conversion circuit 41, the bandpass width is designed to be about 1 to 2 MHz.
Needs for low cost, small capacity, and the like have been remarkably required for the wireless communication terminal apparatus, and an integration of a circuit structuring a terminal has been advanced year after year. However, a problem of a crosstalk of signals or harmonics between circuits has occurred along with an enhancement of the integration. Further, an improvement of a semiconductor process in recent years is being advanced in a direction of decreasing a parasitic capacitance, which also promotes the problem of the crosstalk between circuits. Further, a problem of the crosstalk through a mounting substrate has occurred due to a high density mounting such as an IC in a terminal.
FIG. 8 shows measurement results of the transmitter in which circuits surrounded by solid lines 58 and 59 in FIG. 6 are integrated in the same IC. The GSM is assumed as the system, and a GMSK modulation signal is used as the base band signal. The first frequency synthesizer 38 oscillates at 1080 MHz, is quadrant divided in the divider 47, where the fIF is assumed to be 270 MHz. Further, the fRF (=fIF+fVCO) is set so that the fVCO becomes a GSM (including EGSM) transmission frequency (880 MHz to 915 MHz). The horizontal axis denotes the transmission frequency fVCO of the transmitter, and the vertical axis denotes a worst value of a difference between a signal level at the transmission frequency and a signal level at 400 MHz to 1.8 MHz offset and 6 MHz to 25 MHz offset from the transmission frequency by dB. A spectrum analyzer is used to measure the output of the VCO 57, and the measurement conditions thereof are RBW=VBW=30 kHz at 400 kHz to 1.8 MHz offset, and RBW=VBW=100 at 6 MHz to 25 MHz. The specifications with respect to Spurious emissions of the GSM are not more than −60 dB and not more than −71 dB at 400 kHz to 1.8 MHz offset and at not less than 6 MHz offset, respectively. When the transmission frequency is in the vicinity of 900 MHz at 400 kHz to 1.8 MHz offset, it is found that, when the transmission frequency is in the vicinity of 898 MHz and in the vicinity of 902 MHz at 6 MHz to 25 MHz offset, the transmission spectrum is degraded, which does not meet the GSM specification. This is because undesired spurs occur in the offset frequency shown in formula 1 from the transmission frequency due to an intermodulation of harmonics of the fIF, the fRF and the fVCO.±|3×fVCO−10×fIF|  (Formula 1)Here, a relationship of fVCO=fRF−fIF is present between the fIF, the fRF and the fVCO. The undesired spurs occur as the result that the circuits surrounded by the solid lines 58 and 59 in FIG. 6 are integrated in the same IC so that influences due to the harmonics of the output signals of the first frequency synthesizer 38 and the second frequency synthesizer 39 are increased. Even when the circuits surrounded by the solid lines 58 and 59 are integrated in another IC, there is a possibility that the undesired spurs occur depending on the characteristics of the integrated IC, a semiconductor process to be used, or a method for mounting on a substrate.
Next, FIG. 9 shows an output spectrum of the transmitter in which the circuits surrounded by the solid lines 58 and 59 and the VCO 46 in FIG. 6 are integrated in the same IC. The GSM is assumed as the system, and the GMSK modulation signal is used as the base band signal. The first frequency synthesizer 38 oscillates at 1080 MHz, and is quadrant divided in the divider 47, where the fIF is assumed to be 270 MHz. The fRF is set at 1150 MHz so that the fVCO becomes 880 MHz. Further, the output frequency of the crystal oscillator 40 is 13 MHz. The horizontal axis denotes a frequency, and the vertical axis denotes a signal level by dBm. The measurement is performed by the spectrum analyzer, and the measurement condition is RBW=VBW=30 kHz. The undesired spurs occur at 1 MHz offset from the transmission frequency, and the level thereof is −58.2 dB. As described above, in the GSM specification, the level is prescribed to be not more than −60 dB at 400 kMz to 1.8 MHz offset, and the measurement result in FIG. 9 does not meet the GSM specification. The occurrence process of the undesired spurs is as follows. Harmonics of 1079 MHz which is 83 times the output signal of the crystal oscillator 40 occur in the first frequency synthesizer 38 or the second frequency synthesizer 39. This 1079 MHz signal is mixed into the VCO 46 due to the crosstalk. When the VCO is regarded as a positive feedback circuit, this 1079 MHz signal is amplified in the VCO 46, and at the same time, the undesired spurs also occur at 1081 MHz due to a folded operation around the oscillation frequency 1080 MHz by odd-order nonlinearity characteristics of the amplifier. Details of the folded operation of the noises in the VCO are described in chapter 7.4.3 in Prentice Hall PTR Prentice-Hall, Inc. Press, Behzad Razavi, “RF MICROELECTRONICS” (ISBNO-13-887571-5).
As a result that the present inventors have investigated the transmitter according to the comparative example which is the assumption of the present invention, the followings have become apparent. The transmitter according to the aforementioned comparative example has the problems of the undesired spurs described later due to a progress of the integration of a circuit, a deterioration of the parasitic capacitance due to the improvement of the semiconductor process, or the high density mounting of a terminal.
A first problem (1) is in that the undesired spurs occur at a specific transmission frequency due to the harmonics of the output signal of the frequency synthesizer.
A second problem (2) is in that, when the harmonics of the output signal of the crystal oscillator are present in the vicinity of the oscillation frequency of the VCO, the undesired spurs occur in the VCO output due to the folded operation of the VCO.
Further, since it is difficult to predict the crosstalk between circuits or the crosstalk through a mounting substrate on designing the circuit or the mounting substrate so that it is required that improvements are added after actual manufacture and measurement have been performed, and a large amount of labor and cost has been required.
Therefore, it is a first object of the present invention to solve the problem of the undesired spurs due to the harmonics of the output signal of the frequency synthesizer occurring in the transmitter according to the aforementioned comparative example, and to facilitate to design a circuit or a mounting substrate.
Further, it is a second object of the present invention to solve the problem of the undesired spurs due to the harmonics of the output signal of the frequency synthesizer in the transmitter according to the aforementioned comparative example, and, at the same time, to solve the problem of the undesired spurs occurring when the harmonics of the output signal of the crystal oscillator are mixed into the VCO, and to facilitate to design a circuit or a mounting substrate.
The above and other objects and novel features according to the present invention will be apparent from the description and accompanying drawings of the present specification.